A PTC element often comprises a body of an electrically conductive polymer composition a resistivity with a positive temperature coefficient, the body defining two parallel end surfaces, and two electrodes arranged in contact with the end surfaces for carrying current through the body. The polymer composition of the body includes a polymer material and an electrically conductive powdered material distributed in the polymer material. The expression PTC element is the accepted term for an element whose resistivity has a positive temperature coefficient. PTC elements are used in electric circuits as overcurrent protection.
The resistance of a PTC element of the above-described kind is low, for example a few m.OMEGA., in the normal operating range of the element, which may extend to, for example, 80.degree. C. and increases slightly with the temperature. If the temperature of the element exceeds this value, for example because of an overcurrent, the resistance increases more rapidly, and when exceeding a certain temperature, the element suddenly changes from a low resistance to a high resistance state in which the resistance may amount to one or a few tens of k.OMEGA.. It is well-known that the resistance of the PTC element, after changing from low resistance to high resistance state, does not return to the initial resistance. In more serious cases when it is subjected to very great electrical stresses, such as short-circuit currents, blistering and crack formation occur in central or other parts of the polymer composition of the PTC element, so that the the PTC element can no longer function, that is, the element becomes destroyed.
In the known PTC elements the aim has been to secure the electrodes as efficiently as possible to the body of polymer composition to achieve the best possible electrical contact and hence minimize the contact resistance. To achieve the good securing of the electrodes, they are normally formed with an uneven surface structure on the side facing the body of polymer composition, so that the polymer composition during the manufacturing of the PTC element is able to melt and penetrate into cavities in the electrode surface. Usually, the electrodes consist of metal foils and are applied by being pressed onto the body with heating.
According to the present invention it has proved to be possible to counteract or completely eliminate the abovedescribed unfavourable processes during operation of the PTC elements and to produce a PTC element which, after transition from a low resistance to a high resistance state, returns to the initial resistance, and which is reusable also after having been subjected to short-circuit currents. Further, the PTC element changes its resistance in case of a lower energy development, that is, its current limiting properties are improved.
According to the invention, the above result is obtained by producing at least one electrical contact between an electrode and a body of electrically conductive polymer composition or between two bodies of electrically conductive polymer composition in the PTC element by the electrode making free contact with the mentioned body or that the mentioned bodies make free contact with each other while maintaining a pressure directed perpendicular to the contact surface in question by means of a pressure device.
More particularly, the present invention relates to a PTC element comprising a body, provided with two parallel surfaces, of an electrically conductive polymer composition with a positive temperature coefficient, in which the parallel surfaces on the body of polymer composition or one of the parallel surfaces on the body of polymer composition and one of the parallel surfaces on another body included in the PTC element or on one of several other bodies included in the PTC element and arranged geometrically parallel to the first-mentioned body, which latter body or bodies, respectively, is/are of the same or a different electrically conductive polymer composition with a positive temperature coefficient, are arranged in electrical contact with electrodes, included in the PTC element, for carrying current through the body, or bodies, of polymer composition, characterized in that the body of polymer composition, if only one such body is included in the PTC element, with at least one of its parallel surfaces is adapted to make free contact with one of the electrodes, or at least one of the bodies of polymer composition, if more than one such body is included in the PTC element, with at least one of its parallel surfaces is adapted to make free contact with one of the electrodes or with the other body, or one of the other bodies, of polymer composition, and that a pressure device is adapted to exert on the electrodes a pressure directed perpendicular to the parallel surfaces on the body, or bodies, of polymer composition. The expression "to make free contact" here and in the following means that the respective elements making free contact are not fixed to each other but make contact with each other only by abutment.
A feasible explanation of the result obtained according to the invention may be the following. During normal passage of current, a low contact resistance is maintained between the elements which make free contact with each other due to the pressure exerted on the contact surface. In case of short-circuit currents, electrodynamic repulsion forces occur between the elements making free contact with each other, which leads to a separation of the elements and hence a reduction of the number of contact points between electrode and conducting particles in the body of the polymer composition making free contact therewith or between conducting particles in bodies of the polymer composition making free contact with each other. This leads to a current concentration at the remaining contact points, which causes molten phases to occur in the polymer composition at the contact surface and the PTC element to trip at the contact surface without the rest of the polymer composition being subjected to any stress with ensuing unfavourable effects. Since the pressure against the contact surface remains, when the surface is still hot after the short circuit, the original contact and the original contact resistance may be reestablished between the elements making free contact with one another.
The polymer composition may be of a known kind and its composition or constituents constitute no part of the present invention. Thus, the polymer material may consist of thermoplastic resins, elastomers, thermosetting resins or mixtures thereof used in prior art polymer compositions with PTC behaviour. As examples of suitable polymer materials may be particularly mentioned polyolefins such as polyethylene, crosslinked polyethylene, polypropylene, polybutene and copolymers or ethylene and propylene. The polymer material preferably has a crystallinity of at least 5%. The conducting powdered material preferably consists of conducting carbon black or conducting soot. However, it is possible, per se, to use, together with or instead of the mentioned powdered material, conducting particles of another kind, such as particles of metallic materials, for example nickel, cobalt, copper and silver. A suitable particle size of the powdered material is 0.01-10 .mu.m and a suitable content of the powdered filler is 10-60 percent of the total volume of the powdered filler and the polymer material. The resistivity of the polymer composition is preferably within the range of 10 m.OMEGA.cm-100 .OMEGA.cm and has the ability, after a transition, to exhibit a resistivity of 1 .OMEGA.cm-1 k.OMEGA.cm. If more than one body of electrically conductive polymer composition is included in the PTC element, the bodies may be of the same or different polymer composition and then with the same or different resistivity.
In those embodiments of the present invention in which at least one of the electrodes is firmly secured to a body of conducting polymer composition, such an electrodes or such electrodes may be of a conventional kind. They may consist of metal foils or of thin metal plates or metal nettings which are rolled on the polymer composition in heated state or fixed in some other way thereto, such as by spraying or vapour deposition. Usually, the surface facing the polymer composition has an irregular structure. The electrodes may also consist of a combination of two or more elements, for example a thin foil of nickel and a thicker plate of copper fixed thereto. According to one embodiment of the present invention, such fixed electrodes may consist of a tight plate or foil or metal, which on the side facing the polymer composition is coated with a layer of metal with an irregular surface structure, applied by thermal spraying such as plasma spraying, flame spraying or arc spraying, which metal layer comprises protruding portions with a height of 1-50 .mu.m and a width of 1-50 .mu.m.
In those embodiments of the present invention where at least one of the electrodes makes free contact with a body of conducting polymer composition, such an electrode may in itself, that is, apart from the fact that it is not fixed to the body of conducting polymer composition, be of the same kind as indicated in the preceding paragraph. It may also consist of a more or less thick plate with a surface with an even structure facing the body of conducting polymer composition. It is also possible, per se, to use other high conductivity materials than metallic materials in the electrodes, such as polymer material containing very high contents of electrically conductive powdered fillers, such as conducting carbon black, conducting soot, copper or nickel.
The pressure which is maintained on the electrodes perpendicularly to the parallel surfaces on the body, or bodies, of polymer composition preferably amounts to at least 0.1 MPa (14.7 psi). Especially preferred is a pressure of 0.1 MPa-10 MPa (14.7 to 1470 psi).
The pressure on the electrodes of the PTC element may be achieved purely mechanically or by utilizing forces generated by electric currents. When utilizing purely mechanical forces, the pressure device may, for example, consist of two plates arranged in parallel with the parallel surfaces of the body, or bodies, of electrically conductive polymer composition and arranged outside the electrodes, and of drawing devices or clamping devices arranged in or adjacent the plates. When utilizing forces generated by electric currents, the pressure device may comprise plates of high conductivity material making contact with the electrodes, in which the current paths are adapted to be substantially parallel to the parallel surfaces of the body, or bodies, and directed in substantially the same direction as the plates, or yokes of a ferromagnetic material arranged outside the plates of high conductivity material.
According to a particularly preferred embodiment of the invention, the pressure device is provided with pressure-exerting devices with the ability to be resilient. Such a design of the pressure device significantly facilitates a separation of the electrode and the body of polymer composition making free contact with each other, or of bodies of polymer composition making free contact with each other, at short-circuit currents. In a preferred embodiment the pressure device comprises two pressure-exerting parts, making contact with the electrodes of the PTC element by abutment, and a layer of rubber or another elastic material, for example an elastic plastic, is arranged between one of the electrodes and one of the pressure-exerting parts.
According to another preferred embodiment the pressure device comprises parts exerting pressure against the electrodes of the PTC element, and at least one resilient element arranged between the pressure-exerting parts outside en electrode.
According to an additional embodiment of the invention, a PTC element according to the invention is stacked on top of another PTC element according to the invention in the same pressure device, which is thereby adapted to exert a pressure directed perpendicularly to the parallel surfaces of the body, or the bodies, of polymer composition in each one of the PTC elements, and the outwardly-facing electrodes are electrically parallel-connected as are the inwardly-facing ones, which may possibly be designed to form one single electrode. Preferably, the device is formed with separate inwardly-facing electrodes and with a layer of rubber or other elastic material, such as elastic plastics, arranged between the electrodes. These embodiments with parallel-connected outwardly-facing and inwardly-facing electrodes have very good current-limiting properties also in case of a total area of the PTC elements which is relatively large.
According to yet another embodiment of the invention, the body of polymer composition, if only one such body is included in the PTC element, or the bodies of polymer composition, if more than one such body is included in the PTC element, at surfaces exposed between the electrodes is/are surrounded by a casing of insulating material, which may advantageously be of the same kind and thus have the same physical properties as the polymer material which is included in the polymer composition. The casing of insulating material increases the dielectric strength of the PTC element. The reason is that the casing prevents flash-over between particles of the conducting powdered material, which may be exposed at surfaces of the polymer composition, which are exposed between the electrodes, when the PTC element is subjected to short-circuit currents.
According to a particularly preferred embodiment of the invention, the pressure device comprises a pressure-absorbing frame arranged around the electrodes and the body, or bodies, of electrically conducting polymer composition, with wedge-shaped elements insertable into the frame. The frame may have the shape of an apparatus housing provided with a lid and at least one of the wedge-shaped elements may be arranged in the apparatus housing itself and at least one of the wedge-shaped elements may be secured to the lid.
Another embodiment of the invention is characterized in that the parallel surfaces on the body, or bodies, of electrically conductive polymer composition consist of concentric surfaces. The electrodes are thereby concentric with the mentioned concentric surfaces of the body, or bodies, of polymer composition. This embodiment requires an extremely small space.
A PTC element of the kind mentioned in the preceding paragraph may be designed such that, inside an electrode which is arranged at an inwardly-facing concentric surface on the body, or on that body of polymer composition which is arranged furthest towards the interior, there is arranged a body which is expansible in a direction perpendicular to the concentric surface and that, outside an electrode which is arranged at an outwardly-facing concentric surface of the body, or on that body of polymer composition which is located furthest towards the exterior, there is arranged a counter-support or this electrode is itself designed to constitute a counter-support.